In order to optically perform recording/reproduction of information on an information carrier using a light source, it is necessary to perform focus control such that the information surface of the information carrier is always at the focal point (convergent point) position of the light beam. In order to realize this, prior to focus control, a so-called focus pulling-in action is performed that moves the objective lens and brings the focal position of the light beam to the information surface of the information carrier.
Conventional optical disk control devices attempt to realize a reduction in the size of the optical pickup (for example, see JP H05-334687A) by shortening the distance between the information surface and the objective lens, a so-called working distance (hereinafter, referred to as “WD”).
Below, a conventional optical disk control device will be explained with reference to FIG. 14, FIG. 15, and FIG. 16.
FIG. 14 is a block diagram showing an example of a configuration of a conventional optical disk control device.
In FIG. 14, a sawtooth-shaped wave signal generating circuit 45 outputs a sawtooth-shaped wave signal with gradually increasing amplitude. A switching circuit 31 switches between the output signal of the sawtooth-shaped wave signal generating circuit 45 and the output signal of a control circuit 20 and feeds one of these to an actuator driving circuit 21 as a signal “a”, and the actuator driving circuit 21, by operating an actuator 22 in response to the signal “a”, drives an objective lens 23.
A focus error detecting circuit 12 outputs a focus error signal b that indicates the amount of displacement between the focal point position of the objective lens 23 and an information surface 2A of an optical disk 2, the details of which will be stated below. A focus pulling-in circuit 32F determines the level of the focus error signal b, and by giving an instruction g to the switching circuit 31 a focus pulling-in action is realized.
FIG. 15 is a circuit diagram showing an example of the internal configuration of the focus error detecting circuit 12 in FIG. 14. In FIG. 15, the focus error detecting circuit 12 generates a focus error signal b by an astigmatic method wherein, from a signal detected in response to an incident light beam spot 302 by a quartered photodetector 301 in an optical pickup 3, two adders 1201 and 1202 generate sum signals (A+D) and (B+C), which are the diagonal partial sums of the quartered photodetector 301, and a subtracter 1203 generates a differential signal of (A+C)−(B+D).
Next, the focus pulling action in a conventional optical disk control device configured as described above will be explained with reference to FIG. 16. FIG. 16 is a waveform diagram of the signals in various parts of the optical disk control device of FIG. 14.
When a focus pulling-in instruction h is output from a system controller 30, the objective lens 23 is driven via the actuator driving circuit 21 and the actuator 22, based on a sawtooth-shaped wave signal with sequentially changing amplitude that is output from the sawtooth-shaped wave signal generating circuit 45. The amplitude of that sawtooth-shaped wave signal gradually increases, and when the focal point of the objective lens 23 reaches the information surface 2A, the focus pulling-in circuit 32F determines the level of the focus error signal b that is output from the focus error detecting circuit 12, and at the timing that the level reaches the focus pulling-in level, outputs the switching instruction g to the switching circuit 31. By switching the signal “a” that is output to the actuator driving circuit 21 from the output signal of the sawtooth-shaped wave generating circuit 45 to the output signal of the control circuit 20 along with activating the control circuit 20, a focus pulling-in action is performed.
In this sort of optical disk control device, because the amplitude of the sawtooth-shaped wave signal is gradually increased, it takes time for the focus error signal to be detected, and as a result, time is required for the focus pulling-in action.
In particular, when the space between the optical disk and the objective lens increases due to surface oscillation of the optical disk or drooping of the objective lens, the time until the focus error signal is detected increases. Thus, the time required for the focus pulling-in action is increased further.
There is also the problem that focus pulling cannot be performed on the target information surface when the optical disk has a plurality of information surfaces, because it is not possible to discern to which information surface the detected focus error signal corresponds.
Particularly, because more focus error signals are detected the further the target information surface gets from the objective lens, pulling the focus to the target information surface is difficult